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Next: 46.2.7 Numerical Hessian (NUMHESS) Up: 46.2 Directives for OPTG Previous: 46.2.5 Defining inactive geometry   Contents   Index   PDF


46.2.6 Hessian approximations (HESSIAN)

By default, the MOLPRO geometry optimization utilizes a force field approximation to the hessian (``Model Hessian'', see R. Lindh, A. Bernhardsson, G. Karlström and P. Malmqvist Chem. Phys. Lett. 241, 423 (1995)), which speeds up convergence significantly. The Model Hessian is parameterized for the elements up to the third row. Alternatively, the model Hessian of Schlegel can be used, or the Hessian can be computed numerically (see also section 46.2.7).

HESSIAN,options

where options can be

MODEL
Use Lindh's Model Hessian in optimization (default).
MODEL=SCHLEGEL
Use Schlegel's Model Hessian.
MODEL=VDW
Add vdW terms to Lindh's Model Hessian.
SCHLEGEL
same as MODEL=SCHLEGEL.
VDW
same as MODEL=VDW.
NOMODEL
Don't use Model Hessian approximation to the hessian.
NUMERICAL=hstep
Recompute Hessian after hstep iterations. This disables the use of a model hessian. If hstep=0, the Hessian is only computed in the first iteration. Default parameters are used for computing the numerical Hessian, unless modified using options as described for the NUMHESS directive, see Sect. 46.2.7. Any option valid for the NUMHESS directive may also follow the NUMERICAL option on the HESSIAN directive.
READ|RECORD|HESSREC=record
Read Hessian from given record. If record is not given or zero, the last computed hessian will be read. See section 46.2.7 for more details about numerical Hessians.
UPDATE=type
Method used for hessian update. See section 46.2.9 for possibilities and details.
MAXUPD=maxupd
Max number of hessian updates. The count is reset to zero each time a hessian is computed.

If the Model Hessian is disabled (NOMODEL) and no Hessian is read or computed, the initial hessian is assumed to be diagonal, with values 1 hartree*bohr**(-2) for all lengths, 1 hartree*radian**(-2) for all angles. Additional matrix elements of the hessian can be defined using the HESSELEM directive, see section 46.2.8.

In transition state searches the Hessian is evaluated numerically in the first iteration by default. Alternatively, if READ is specified, a previously computed hessian is used.



Next: 46.2.7 Numerical Hessian (NUMHESS) Up: 46.2 Directives for OPTG Previous: 46.2.5 Defining inactive geometry   Contents   Index   PDF

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